Food Lipids: Chemistry, Nutrition, and Biotechnology

hydrogen and lipid peroxides to form radicals [27, 28]. Ascorbate also causes the
release of iron, which is sequestered to proteins such as ferritin [29]. Therefore,
ascorbate and glutathione can potentially exhibit prooxidative activity in the presence
of free transition metals or iron-binding proteins. In addition, in the presence of
oxygen, glutathione radicals are capable of forming high energy peroxides that can
potentially catalyze the oxidation of lipids [3].
Thiols besides glutathione can inactivate free radicals. Cysteine is capable of
scavenging free radicals. The energy of the resulting thio radical is high, however,
suggesting that it may promote oxidation [3]. Thioctic acid is another thiol that can
inactivate peroxyl radicals [30]. However, the reduced state of thioctic acid, dihydrolipoic
acid [31] and cysteine [32], can be prooxidative because their reducing
potential, and thus their ability to stimulate metal-catalyzed oxidation, is strong.
Several nitrogenous compounds can inactivate free radicals. Uric acid inactivates
both hydroxyl and lipid radicals and inhibits lipid oxidation at physiological
concentrations [33]. Uric acid is an important antioxidant in blood plasma [33–35].
Since uric acid is produced in postmortem skeletal muscle via ATP metabolism [36],
it might possibly serve as an active endogenous antioxidant in muscle foods.
Amino acids, peptides, and proteins can interact with free radicals. Amino
acids, including histidine, tyrosine, phenylalanine, tryptophan, cysteine, proline, and
lysine, are capable of inactivating free radicals [37–40]. Blood proteins have been
estimated to provide 10 to 50% of the peroxyl radical trapping activity of plasma
[34,41]. Serum albumin scavenges carbon-based free radicals partially through the
involvement of its free sulfhydryl groups [42]. Amino acids, peptides, and proteins
have been reported to inhibit lipid oxidation in bulk and emulsified lipid systems as
well as in food products [43–47].
While protein, peptides, and amino acids often possess the structural characteristics
needed to both scavenge radicals and inhibit lipid oxidation, the concentrations
required for activity are often higher than other free radical scavengers. This
suggests that free radicals inactivated by proteinaceous compounds do not act as
chain-breaking antioxidants but instead are simply competing with the lipid for high
energy radicals. Interactions of free radicals with amino acids and proteins leads to
the formation of peroxides [37,48]. However, very little is known about how the
formation of amino acid peroxides or other amino acid oxidation products influences
the antioxidant activity of amino acids, peptides, and proteins.
III. CONTROL OF LIPID OXIDATION CATALYSTS
Lipid oxidation rates in foods often depend on catalyst concentrations and activity.
Control of lipid oxidation catalysts can therefore be a very important factor in controlling
oxidative rancidity. Both endogenous and added antioxidants help control
the activity of transition metals, singlet oxygen, and enzymes.
A. Control of Prooxidant Metals
Transition metals accelerate lipid oxidation reactions by hydrogen abstraction and
peroxide decomposition, resulting in the formation of free radicals [26]. The activity
of prooxidative metals is influenced by chelators or sequestering agents. Transition
metals such as iron exhibit low solubility at pH values near neutrality [25]. Therefore,
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